Modulation system



0- K. SCHLESINGER 27,227,027

' MODULATION SYSTEM Filed Dec. 15, 1957 2 Sheets-Sheet 1 Fly. 2

Inventor? Dec. 31, 1 K. SCHLESINGER MODULATION SYSTEM 2 Sheets-Shet 2 Filed Dec. 15, 1937 Inventor? Patented Dec. 31, 1940 UNITED. STATES PATENT OFFIQE.

MODULATION SYSTEM Kurt Schlesinger, Berlin, Germany, assignor, by mesne assignments, to Loewe Radio, Inc., a corporation of New York Application December 15, 1937, Serial No. 179,932 In Germany December 21, 1936 12 Claims.

as large as possible. Accordingly in circuits of thisnature tuning condensers have been omitted and the tuning is performed by variation of the self-induction, i. e., in the form of variometer tuning. The latter, apart from the drawbacks in construction associated with the very small 15 variometer required, has the disadvantage that upon the change over to shorter waves the oscillation resistance is reduced together with the inductance. Particular disadvantages occur when circuits of this nature are called upon to conduct a mixture of ultra-high frequencies and image frequencies, as is the case, for example, in the modulation of television transmitters. So long as the coil and the tube are connected in parallel a feed condenser must be employed, which has infinitesimal resistance with respect to the ultrahigh frequency. A condenser of this kind, however, practically short-circuits the modulating potential.

The present invention describes a method of 3 carrying out circuits of this nature with variable tuning, in which the oscillation resistance does not decline upon a change over to shorter waves, the detrimental capacities are always smaller than, or at the most equal to, the tube capacities, and the coil is able to receive a substantially reater inductance than in the case of the said known methods. The admixing of an image frequency in the circuit is also attained Without detrimental short-circuiting of this potential source.

The present invention will be clearly understood by way of examples shown in the accompanying drawings, wherein 5 Fig. 1 shows a well known normal circuit arrangement,

Fig. 2 shows a circuit arrangementuin accordance with the invention, and

Figs. 3-5 show further embodiments of the fundamental idea of the present invention.

In Fig. 1 the oscillatory circuit comprising the coil I and the tuning capacity 2 is applied normally in the case of long waves parallel to the grid cathode circuit of a tube. If the gridcathode (control electrode-cathode) capacity is C3, the circuit requires a damping resistance R4 amounting to wherein Xe is the reactance The damping must accordingly have one-half of the resistance value of the capacitative impedance. In this circuit a: is the maximum angular frequency of the image to be transmitted, i. e. the Width of band of the image. Owing to the parallel connection of the capacities 2 and 3 the damping resistance is reduced in Very unfavourable manner. The condenser 5 assumes the blocking of the circuit to earth. If the tube 3 is to be modulated, for example by an image change potential 6, this potential source is practically short-circuited by the condenser E. A large output is accordingly consumed.

According to the invention, the tuning of an oscillatory circuit and its connection to the tube is performed by series resonance in accordance with Fig. .2. In this connection the coil I can be selected to have three to four times the inductance as compared with the parallel connection in Fig. 1. It is so adjusted that its natural wavelength together with the grid cathode capacity C3 of the tube is equal to the longest wave of the tuning range in the case of the tuning condenser 2 being short-circuited. The condenser 2 is selected, as a maximum, to be somewhat greater than C3. As regards the minimum it is less than C3. When tuning according to short waves by the reduction of condenser 2 the ratio improves more and more from its most unfavourable value, viz.,

to more favourable values, as the effective capacity is reduced by series connection of the tuning condenser 2 and the tube capacity C3. The damping resistor 4, which is applied parallel to the coil, can accordingly be of a much more highly ohmic kind, viz., according to the above band width formula, and at least equal to -Xc, i. e. equal to one-half of the value of the reactance of the tube capacitator 3 even in respect of the highest modulating frequency. A more favourable result is incapable of being obtained with any other connection system in the case of a tube with C3.

The circuit system can naturally also be employed in similar fashion in circuits coupling the grid and anode respectively.

Fig. 3 shows the circuit for grid modulation of a transmission tube. The control transmitter l excites inductively the grid coil I. This is selected to be so large that it is in resonance with the grid-cathode capacity C3 of the transmission tube 8 on the longest wave of the range. Since the tuning condenser E is merely of the order of the grid-cathode capacity C3 (approximately 20 m.m.f.), it is traversed by the modulating potential 6 without great expenditure of output ,even in the case of comparatively high image frequencies. The bias is supplied by way of a choke 9 or a reactance which is small as compared with the impedance of 2.

Fig. 4 shows the use of the series resonance in the case of a transmitter connected in push-pull.

Two tubes 8a, 8b are therein provided. The grid circuit is not directly connected with the earthed cathode. It is accordingly earthed only by way of the two grid-cathode capacities 3a, 31). Since these are in series, the detrimental capacity is merely equal to one-half of the single capacities. The useful inductance la, lb is even twice as great as in Fig. 3. The detrimental short-circuiting of the modulating potential 5 by the timing condenser E is done away with entirely, as the modulating potential is now loaded merely by the total capacity Sad-3b, and in no way loaded by the tuning capacity 2, as this is present only in unipolar fashion in respect of the modulating potential. Moreover, in the push-pull connection the advantage of the series resonance can also be employed in the anode circuit. The existence of the push-pull stage permits of the neutralisation by way of the condensers lea, ltb, which is possible in the single stage according to Fig. 3 only by means of artificial circuits. The anode circuit coils I la and Hbare tuned together with the anode-earth capacities of the tube 8 to the longest wave of the range and are detuned to shorter waves by the twin condenser 12. The

of this connection is already so good that in the case of LOO-line images dampings of more than 5,000 ohms have been found to be adequate.

Despite the use of the series resonance it may occur that the natural band width transmitted by this circuit is still not suflicient. This is the case in a sound and image receiver. Thefirst tuning circuits thereof must cover the entire modulating band width of the image transmitter and in addition the frequency requirement of the sound transmitter and its frequency spacing from the former. This results in the case of 400-line transmission systems in band widths of between 5 and 6 megacycles, i. e. approximately 15% of the carrier frequency. In this case the principle of series resonance according to the invention is extended to a band filter connection, as shown in Fig. 5. A band filter according to the invention for intermediate tube coupling accordingly comprises, for example, two coils la, lb, which result in conjunction with the anode or grid capacity of the tubes to be coupled in the longest wave of the range. The coils are inductively coupled, but are so poled that grid potential and anode potential are transmitted in 00- phasal fashion, i. e. for example both negatively at the moment concerned. The two coils have anadditional coupling by reason of the natural capacity I3 between their windings. This capacitative coupling acts in an assisting sense and prevents a decrease in the width of band upon tuning to shorter waves. The tuning takes place by means of two condensers Illa, 5223, which may also be disposed about the same axis assuming the tube capacities are equal. The latter may be adjusted in respect of the smaller of the two by a very small auxiliary capacity i l. Generally speaking, damping means' are not required if the coupling is sufficiently firm. A system of this kind with values of the capacities l2 amounting to 20 to 5 m. m. f. and with inherent capacities of the tubes amounting to approximately 10 m. m. f. covers the tuning range between 5 and 8 metres with constant width of band of approximately 5 megacycles. Without the capacity [3 the width of band between 8 and 5 metres drops from 5 to 3 megacycles. With utilisation of the electro-static coupling i3 it remains practically constant, and with reverse polarity of one of the two coils the dependency is increased, the width of band being particularly small at the shortwave end and particularly large at the long-wave end. The tube coupling shown in Fig. 5 can naturally also be employed as coupling between aerial and first tube.

The tuning connection according to the invention is of particular importance in the pre-selection of television receivers. The combination of two tuning circuits connected in accordance with the invention to form a band filter with an aperture of approximately 5 megacycles width, as set forth in Fig. 5, always has the known increase of the degree of transmission at the two flanks as compared with a transmission reduced by approximately 50% in the saddle of the frequency curve. This increase at the flanks might be overcome by resistances ta, db connected in parallel to the two coils la, lb. It is more convenient, however, to combine the above disclosed band filter which is used between the tubes, with a simple oscillatory grid circuit having a tuning circuit connection l5, it. The aerial is coupled to the coil l5 by way of a primary coil H which has no resonance effect. This forms with the tuning condenser [E the oscillatory circuit system according to the invention, the coil having an inductance which is such that it resonates with the grid-cathode capacity of the tube 3 to the longest wave of the range. The resonance curve of this grid circuit is a simple bell curve, which is so applied that its maximum coincides with the saddle of the following band filter la, lb between the tubes. The advantage is high amplification on the carrier wave, rattling noises, audible with high frequency.

In order in the case of single-knob control also to be able to couple an oscillator frequency to a tuning structure of this kind comprising three condensers l6, 12a, l2b, it is necessary to provide the oscillator I8 with a tuning curve similar to that in the selection circuits. For this purpose the same tuning method according to the invention can be employed with a shortening capacity IS, the two coil halves 20a, 2% forming together with IS and the grid-anode capacity of E8 the local oscillation circuit. The natural wave length of this circuit is reduced by way of 19. In this case it is possible to mount all four condensers l6, l2a, I21) and I9 on a single shaft. The condensr l9 may also be in the form of a differential mid-point with the cathode,

, electron discharge device having at least a cathode, a control electrode and. an anode, a serially connected inductance and a variable capacity element, said variable capacity element having a mean capacity value substantially equal to the inter-electrode capacity between the control electrode and the cathode of the electron discharge device, mans to connect the serially connected inductance and variable capacity element between the control electrode and the cathode of the electron discharge device, said inductance having a value such that in combination with the inter-electrode capacity between the control electrode and the cathode of the discharge device and the variable capacity element the natural period of the tuned circuit may be varied between the predeterminedupper and lower elements of frequency.

2. In a high frequency radio circuit designed to operate over a relatively wide band of frequencies between a predetermined lower limit of frequency and a predetermined upper limit of frequency, the.. combination whichincludes an electron discharge device having at least a cathode, a control electrode and an anode, a serially connected inductance and a variable capacity element, said variable capacity element having a mean capacity value substantially equal to the inter-electrode capacity between the control electrode and the cathode of the electron discharge device, said inductance having a value such that in combination with the inter-electrode capacity between the control electrode and the cathode of the discharge device and the variable capacity element the natural period of the tuned circuit may be varied between the predetermined upper and lower elements of frequency, and damping means connected in parallel with the said inductance element.

3. In a high frequency radio circuit designed to operate over a wide band of frequencies between a predetermined lower limit of frequency and a predetermined upper limit of frequency, the combination which includes an electron discharge device having a cathode, a control electrode and an anode, a serially connected inductance and variable condenser, said variable condenser having a predetermined minimum value of capacity smaller than the inter-electrode capacity of the discharge device between said control electrode and said cathode and having a predetermined maximum value of capacity greater than the said inter-electrode capacity between said control electrode and cathode, means to connect the serially connected inductance and variable condenser between said control electrode and said cathode, said inductance having a value such that in combination with the inter-electrode capacity between the control electrode and cathode and the maximum Value of the variable condenser the natural period of the tuned circuit is at the predetermined low-er limit of frequency, and further that in combination with the inter-electrode capacity between the control electrode and cathode and the minimum value of said variable condenser the tuned circuit has its natural period at the predetermined upper limit of frequency.

4. A high frequency radio circuit designed to operate over a relatively wide frequency hand between two predetermined limiting frequencies comprising a pair of substantially like characteristic thermionic tubes-each having a cathode, a control electrode, and an anode, said tubes each having a balanced input circuit between the control electrode and cathode, a series connected inductance and avariable capacity element connected between the control electrode and cathode of each of the tubes, means to supply energy to the input circuit of each of the tubes, said variable capacity having a mean value approximately equal to the inter-electrode capacity between the control electrode and the cathode of the thermionic tubes, said inductance elements each being of a value such that in com-' bination with the serially connected variable capacity element and the said tube capacity the natural resonant period of each input circuit may be varied by the variable capacity element between the said limiting frequencies, and a load circuit for each tube.

5. A high frequency radio circuit designed to operate over a relatively wide frequency band between two predetermined limiting frequencies comprising a pair of substantially likecharacteristic thermionic tubes each having a cathode, a control electrode, and an anode, said tubes each having a balanced input circuit between the control electrode and cathode, a series connected inductance and a variable capacity element connected between the control electrode and cathode of each of the tubes, means to supply energy to the input circuit of each of the tubes, said variable capacity having a mean value approximately equal to the inter-electrode capacity between the control electrode and the cathode of the thermionic tubes said inductance elements each being of a value such that in combination with the serially connected variable capacity element and the said tube capacity the naturalresonant period of each input circuit may be varied by the variable capacity element between the limiting frequencies, and a load circuit, a balanced output'circuit for the two tubes to energize the load circuit, and neutralizing means connected between the anode of each tube and the control electrode of the other tube.

6. A high frequency radio circuit designed to operate over a relatively wide frequency band be- ,tween two predetermined limiting frequencies comprising a pair of substantially like characteristic thermionic tubes each having a cathode, a control electrode, and an anode, said tubes each having a balanced input circuit between the control electrode and cathode, a series connected inductance and a variable capacity element'connected between the control electrode and cathode of each of the tubes, means to supply energy to the input circuit of each of the tubes, said variable capacity having a mean value approximately equal to the inter-electrode capacity between the control electrode and the cathode of the thermionic tubes, said inductance elements each being of a value such that in combination with the serially connected variable capacity element and the said tube capacity the natural resonant period of each input circuit may be varied by the variable capacity element between the limiting frequencies, and a load circuit for each tube,and tuning means in the output circuit of each of said tubes.

7. A high frequency radio circuit designed to operate over a relatively wide frequency band between two predetermined limiting frequencies comprising a pair of substantially like characteristic thermionic tubes each having a cathode, a control electrode, and an anode, said tubes each having a balanced input circuit between the control electrode and cathode, a series connected inductance and a variable capacity element connected between the control electrode and cathode of each of the tubes, means to supply energy to the input circuit of each of the tubes, said variable capac ity having a mean value approximately equal to the inter-electrode capacity between the control electrode and the cathode of the thermionic tubes, said inductance elements each being of a value such that in combination with the serially connected variable capacity element and the said tube capacity the natural resonant period of each input circuit may be varied by the variable capacity element between the said limiting frequencies, and a load circuit, a balanced output circuit for the two tubes to energize the load circuit, neutralizing means connected between the anode of each tube and the control electrode of the other tube, and tuning means in the balanced output circuits.

8. In a high frequency radio circuit designed to operate over a wide band of frequencies between a predetermined lower limit of frequency and a predetermined upper limit of frequency, the combination which includes an electron discharge device having a. cathode, a control electrode and an anode, a serially connected inductance and variable condenser, said variable condenser having a predetermined minimum value of capacity smaller than the inter-electrode capacity of the discharge device between said control electrode and said cathode and having a predetermined maximum value of capacity greater than the inter-electrode capacity between said control electrode and cathode, means to connect the serially connected inductance and variable condenser between said control electrode and said cathode, said inductance having a value such that in combination with the inter-electrode capacity between the control electrode and cathode and the maximum value of the variable condenser the natural period of the tuned circuit is at the predetermined lower limit of frequency, and further that in combination with the inter-electrode capacity between the control electrode and cathode and the minimum value of said variable condenser has its natural period at the predetermined upper limit of frequency, a band-pass filter having an input and output circuit, means to connect the input circuit of said band-pass filter between the anode and cathode of said electron discharge device, av second electron discharge device having at least a cathode, a control electrode and an anode, a second variable condenser means to connect the output circuit of said band-pass filter between the control electrode of said second electron discharge device and said second variable condenser, means to connect said second variable condenser to the cathode of said second electron discharge device, said second variable condenser having a maximum value of capacity greater than the interelectrode capacity between the control electrode and cathode of said second electron discharge device and a minimum value of capacity smaller than the inter-electrode capacity between the control electrode and cathode of said second electron discharge device.

9. In a high frequency radio circuit designed to operate over a wide band of frequencies between a predetermined lower limit of frequency and a predetermined upper limit of frequency, the combination which includes an electron discharge device havinga cathode, a control electrode and an anode, a serially connected inductance and variable condenser, said variable condenser having a predetermined minimum value of capacity smaller than the inter-electrode capacity of the discharge device between said control electrode and said cathode and having a predetermined maximum value of capacity greater than the inter-electrode capacity between said control electrode and cathode, means to connect the serially connected inductance and variable condenser between said control electrode and said cathode, said inductance having a value such that in cornbination with the inter-electrode capacity between the control electrode and cathode and the maximum value of the variable condenser the natural period of the tuned circuit is at the predetermined lower limit of frequency, and further that in combination with the inter-electrode capacity betweenv the control electrode and cathode and the minimum value of said variable condenser has its natural period at the predetermined upper limit of frequency, a band-pass filter having an input and output circuit, means to connect the input circuit of said band-pass filter between the anode and cathode of said electron discharge device, a second electron discharge device having at least a cathode, a control electrode and an anode, a second variable condenser means to connect the output circuit of said band-pass filter between the control electrode of said second electron discharge device and said second variable condenser, means to connect said second variable condenser to the cathode of said second electron discharge device, said second variable condenser having a maximum value of capacity greater than the inter-electrode capacity between the control electrode and cathode of said second electron discharge device and a minimum value of capacity smaller than the inter-electrode capacity between the control electrode and cathode of said second electron discharge device and a damping means connected in parallel with the band-pass filter input and output.

10. In a high frequency radio circuit designed to operate over a wide band of frequencies between av predetermined lower limit of frequency and a predetermined upper limit of frequency, the combination which includes an electron discharge device having a cathode, a control electrode and an anode, a serially connected inducr tance and variable condenser connected between the control electrode and the cathode, said variable condenser having a mean value of capacity approximately equal to the inter-electrode capacity between said control electrode and said cathode of said discharge device and means to connect the serially connected inductance and variable condenser between said control electrode and said cathode, said inductance having a value such that in combination with the inter-electrode capacity between the control electrode and cathode and the maximum value of the variable condenser the natural period of the timed circuit is at the predetermrined lower limit of frequency, and further that in combination with the interelectrode capacity between the control electrode and cathode and the minimum value of said variable condenser the natural period of the tuned circuit is at the predetermined upper limit of frequency, a band-pass filter having an input and output circuit, means to connect the input circuit of said band-pass filter between the anode and cathode of said electron discharge device, a second electron discharge device having at least a output energy from said second electron discharge device.

11. In a high frequency radio circuit designed to operate over a wide band of frequencies between a predetermined lower limit of frequency and a predetermined upper limit of frequency, the combination which includes a thermionic tube having a cathode, a grid and an anode, a serially connected inductance element and a variable condenser element, said variable condenser having a predetermined mean value of capacity substantially equal to the inter-electrode capacity between said grid and said cathode of said tube, means to connect the serially connected inductance and variable condenser between said grid and said cathode, said inductance having a value such that in combination with the inter-electrode capacity of the grid and cathode and the maximum value of the variable condenser the natural period of the tuned circuit is at the predetermined lower limit offrequency, and further that in combination with the inter-electrode capacity of the grid and cathode and the mini'm-um value of said variable condenser has its natural period at the predetermined upper limit of frequency, a second thermionic tube also having a cathode, grid and anode, a second inductance, said inductance being connectedbetween said variable condenser and the grid of said second tube, inductive means to supply energy to both of said induct'ances, means to supply radio frequency energy across said variable condenser, resonant means for supplying input energy from said first named electron discharge device to the input of said second electron discharge device, and means to derive output energy from said resonant means.

12. In a high frequency radio circuit for operation over a relatively wide band of frequencies between predetermined lower and upper limits, two push-pull connected thermionic tubes each of substantially like characteristic including at least a cathode, a control electrode, and an anode, a series circuit comprising an inductance, a variable capacity, and a second inductance connected between the ontrol electrodes of said thermionic tubes, means to supply modulation energy to said inductance elements, a connection to ground potential for each cathode of the said tubes, said variable condenser having a capacity value substantially equal to the inter-electrode capacity between the control electrode and cathode of each tube and having a midpoint thereof substantially at radio frequency ground potential, said inductance being of such value that variation in the capacity value of said condenser between minimum and maximum provides for varying the natural period of the tuned circuit between the said predetermined limiting frequencies and an output circuit for said tubes.

KURT SCI-ILESINGER. 

